In addition to the filtering effect provided by clean room ventilation systems, such systems are generally designed to reduce turbulence by providing a generally laminar airflow pattern. In a laminar flow clean room, air typically flows downward from the ceiling to the floor so that a contaminant particle, such as might be shed by a worker, would be drawn directly downward to be removed by a filter. Thus, such a particle would spend no more than a few seconds in the clean room. Where the airflow is turbulent, there are eddies that create localized upward airflow currents. A contaminant particle might be trapped in such an eddy for up to several minutes. Consequently, turbulent airflow generally results in a higher concentration of contaminants at any given time and creates a greater likelihood that such contaminants might come to rest on sensitive equipment or materials.
Perforated diffuser panels are commonly used in clean room ceilings to reduce turbulence and to provide a generally laminar airflow. Such diffuser panels are typically positioned below ceiling-mounted filter elements, preferably with at least a small space in between. Although air may exit the filter element at uneven pressures across the element, the diffuser panel creates a small backpressure which equalizes the air pressure in the space above the panel. The air then passes through perforations in the diffuser panel, with the perforations acting as point sources having generally equal flow rates. Although there is turbulence immediately below the diffuser panel due to the "nozzle effect" of the perforations, such turbulence is quickly dissipated. Typically, such turbulence becomes negligible at a distance below the diffuser panel equal to several times the diameter of the perforations.
However, while a single diffuser panel can provide an effectively laminar airflow away from boundaries or obstructions, clean room ventilation systems are typically constructed with multiple filter elements mounted in a grid system to permit modular manufacturing and assembly. In such a system, each filter element is separated from adjacent filter elements by a "dead zone" occupied by a grid structural element creating an obstruction to air flow with a corresponding airflow dead zone immediately below. As a result, air flowing downward from the periphery of a filter element tends to be drawn into the airflow dead zone and form a turbulent vortex having an upward airflow beneath the grid element. A rule of thumb predicts that a turbulence zone extends downstream of an obstacle by a distance of about four times the obstacle's width. Thus, in typical systems having a frame or grid element with a width of two to three inches, the turbulence zone may extend 12 inches below the ceiling surface, substantially impairing the clean room function.
An existing approach to reduce the turbulence between filter elements is to extend a V-shaped shroud beneath the grid element. Such a shroud is usually in the form of a "tear drop" fluorescent light fixture cover, and functions like the trailing edge of an airplane wing, allowing the laminar flow regions from adjacent filter elements to rejoin more smoothly with reduced turbulence. This approach typically reduces the turbulence zone to within about 7 inches of the ceiling surface, an improvement, but still problematic. In addition, tear drop light fixtures have the disadvantage of further reducing ceiling height, which is typically at a premium due to the substantial ducting and equipment required above the ceiling. Also, such light fixtures create an obstacle to modular sub-dividing walls that preferably hang immediately below the ceiling surface without a substantial gap. Other concerns in clean room ventilation systems include the difficulty of removing and replacing components of the system, the difficulty of installing very large systems, particularly at the junctions of subsystems, and the expense of stocking replacement filters when different types of filters are used in a particular facility. Also, there are disadvantages that are proportional to the velocity passing through the filters. These include particle shedding, reduced filter life, and noise due to pleat vibration and flutter, as well as the energy and equipment cost associated with the air flow resistance or pressure drop of the filter.
From the foregoing it will be recognized that there is a need for an air diffuser panel that overcomes these drawbacks of the prior art by providing an increased or directional airflow at the periphery of the diffuser panels to fill in the dead space below the structural elements and to create a balanced net airflow that becomes generally laminar within close proximity to the ceiling surface. This need includes the provision of a large filter area per unit area of the clean room, easily removable and replaceable components, and a uniform airflow to minimize contamination. The present invention satisfies this need.
The foregoing additional features and advantages of the present invention will be more readily apparent from the following detailed description which proceeds with reference to the accompanying drawings.